1. Technical Field
The present invention relates to an apparatus for detecting a read error of an optical recording medium which includes a clock track, along with data track(s), having a predetermined pattern of clock bits recorded thereon.
2. Background Art
An optical recording medium has been attracting a good deal of public attention, which has a data recording surface having optical alteration patterns, such as light and dark pits, formed thereon to represent digital data. The optical recording medium realizes a large capacity of memory because of its high density data recording capability. This led us to a consideration of not only a disc type memory but also a card type memory.
In the card type memory or optical memory card, recording is done by forming optical (or magnetooptical) alternations in response to data to be recorded at discrete spots on the surface thereof, and then a light beam, such as a laser beam, is projected thereonto to read the alterations.
Specifically, fine unevenness or light and dark patterns terns are provided on a surface of a recording medium or card onto which a light beam is projected to recognize the recorded data according to the difference of reflectivity, refractive index, or transmissivity of the medium to the projected light beam. Alternatively, the recording medium may be photothermo-magnetically recorded and the recorded data may be read depending upon the change of polarization of the projected light due to the magnetooptical effect.
In juxtaposition with each of the data tracks of such an optical memory card there are provided a clock track for synchronization in a data write/read operation and a tracking guide line as a reference for causing the optical beam of an optical reading system to follow the track accurately.
The write/read timing of the optical recording medium is conventionally determined by a clock signal derived from a detector dedicated for detecting clock bits of the clock track.
Incidentally, if the clock is not accurately read in reading the clock track of the optical recording medium by reason of dust, scratch or the like on the optical recording medium or fluctuations of the moving velocity of the recording medium relative to the optical reading system, then it will be difficult to accurately achieve the write/read operation. An absence of the clock during a short term or relatively slow fluctuations of the velocity can be dealt with by using a phase locked loop (PLL) circuit which produces a clock of a frequency synchronized with the input of the read out or raw clock.
FIG. 5 illustrates a typical arrangement of a prior art PLL circuit. The PLL circuit 66 comprises a phase comparator 52, a low pass filter (LPF) 54, an amplifier 56, a voltage controlled oscillator (VCO) 58 and a 1/N frequency divider 53. This circuit compares, in phase, a received input signal with an output of the divider 53 so as to match the phase through controlling the VCO 58, thus producing, at the output thereof, a clock of a frequency of N times of that of the input signal.
However, for a long-term absence of the raw clock or for an abrupt change of the relative moving velocity of the medium the PLL circuit could not ensure the accurate data reading and may fall into an out-of-locked state.